The study will characterize the biology of FMT in the context of IBS prior to wider clinical application of the method. Given the ethical concerns of unknown and long-term adverse effects of FMT therapy, the study will include patients only with moderate to severe symptoms. IBS is a heterogenous disorder and it is important to characterize the patients, and study as homogeneous patient populations as possible. Therefore the study will only include post-infectious IBS patients after the Giardia outbreak.